The present invention relates to a valve actuation system for providing added motion in the valve mechanism of an internal combustion engine with at least one inlet valve and at least one exhaust valve, the valve mechanism comprising a rotatable camshaft with a cam curve including rising and falling ramps designed to interact with a valve control plunger being movable inside a cylinder for actuation of the inlet or exhaust valve under the action of a valve spring, the control plunger cylinder communicating with a hydraulic fluid feed line and a hydraulic control circuit allowing a return movement of a valve to be delayed in relation to the timing of the corresponding falling ramp.
In internal combustion engines for vehicles it is sometimes desirable to have the facility for switching between different operating modes. For example, it is possible to switch between a conventional symmetrical cycle and an asymmetrical cycle, such as a so-called Miller cycle, by varying the timing of the inlet valve closure during the engine's induction stroke. The advantage in being able to switch between these different operating modes lies, for example, in the ability to vary the effective compression ratio of the engine in order to optimize efficiency and reduce exhaust emissions to a minimum. For this purpose, therefore, variable valve actuation is necessary.
One known method to achieve variable valve movements is by means of mechanical-hydraulic systems of the “added motion” type, e.g. described WO 2004/005677. According to this document, the closing phase of a valve can be delayed for an adjustable period of time counteracting the closing force of the valve spring. A two-way hydraulic control valve is used for activating the added motion function and for controlling engine valve closing.
One disadvantage of such a system is that it is not particularly suitable for providing delayed engine valve closing in part-lift position or multiple stages. It can sometimes be desirable to delay engine valve closing in another part-lift position, other than a position that corresponds to its maximum lift. In the prior art system, an attempt to close the two-way hydraulic valve when the engine valve is well on its way towards closing, would cause intense oscillations in the system if that hydraulic valve is a simple ON-OFF valve. Utilising a proportional hydraulic control valve may solve the shock wave and uncontrollable oscillations problem, but that would drastically increase the cost of the system and its control complexity.
It is desirable to provide inexpensive means for improved flexibility in the control of the closing phase of an engine valve.
According to an aspect of the present invention, a valve actuation system for providing added motion in the valve mechanism of an internal combustion engine with at least one inlet valve and at least one exhaust valve is provided, the valve mechanism comprising a rotatable camshaft with a cam curve including rising and falling ramps designed to interact with a valve control plunger being movable inside a cylinder for actuation of the inlet or exhaust valve under the action of a valve spring, the control plunger cylinder communicating with a hydraulic fluid feed line via a check valve and with a hydraulic control circuit allowing a return movement of a valve to be delayed in relation to the timing of the corresponding falling ramp, which is characterized in that the hydraulic control circuit comprises a first and a second hydraulic fluid flow restrictor connected to the control plunger cylinder at different levels along its longitudinal axis, and that a selector valve enables a connection between the cylinder and the hydraulic control circuit to be opened via either one only or both flow restrictors. This arrangement of the valve actuation system makes it easier to decelerate the engine valves during their closing phase before stopping at an intermediate position.